1 . Field of the Invention
The present invention relates to a hydraulic system for a vehicle, and more particularly, to a hydraulic system for driving both an air conditioning compressor and a power steering mechanism.
2 . Description of the Related Art
Some vehicles with the power steering mechanisms are designed so that the hydraulic system that drives the power steering mechanisms also drives the vehicle's air conditioning compressor. This arrangement simplifies the internal structures of the vehicle's hydraulic system and requires less energy to operate.
A hydraulic system of this type is disclosed in Japanese Unexamined Patent Publication No. 62-12427. In this system, as shown in FIG. 7, a hydraulic motor 101 for driving an air conditioning compressor 100 is connected to a hydraulic pump 103. A power steering mechanism 102 is also connected to the hydraulic pump 103 in parallel with the hydraulic motor 101.
The hydraulic pump 103 is driven by an engine and is connected to two (first and second) branched fluid bypasses 104 and 105. A variable flow rate control valve 106 and a relief valve 107 are located in the first and second fluid bypasses 104 and 105 respectively. A controller 108 controls the opening and closing of the flow control valve 106 based on a signal which reflects various driving loads in the air conditioning system.
In the above-described structure, the pressure downstream of the relief valve 107 becomes low when the power steering mechanism 102 is not activated. The relief valve 107 blocks the second fluid bypass 105, and the pressurized fluid flows to the first fluid bypass 104.
When the power steering mechanism 102 is activated and the internal pressure of the pipe positioned downstream of the relief valve 107 rises, the valve 107 opens. As a result, the pressurized fluid from the hydraulic pump 103 will be supplied toward the power steering mechanism 102 without flowing through the first fluid bypass 104.
This hydraulic system developed on the premise that the power steering mechanism works only when the steering wheel is operated while the vehicle is running slowly or is stopped. If the power steering mechanism 102 is activated only under the above limited conditions, therefore, a change in the operational state of air conditioning system is allowable. However, when the power steering mechanism 102 and the compressor 100 are driven at the same time, the following shortcoming would occur.
The power steering mechanism 102 requires much less pressurized fluid for its operation than the capacity of the hydraulic pump 103. If the amount of pressurized fluid discharged from the hydraulic pump 103 is conformed to the specifications of the power steering mechanism 102, an insufficient amount of pressurized fluid will be available to the hydraulic motor 101. The capacity of the hydraulic motor 101 may be reduced to raise the rotational speed of the compressor 100. However, this measure will result in insufficient power of the compressor 100.
On the other hand, if the amount of fluid from the hydraulic pump 103 is conformed to the rating of the hydraulic motor 101, an excessive amount of fluid will be discharge to the power steering mechanism 102 when the compressor 100 is not working, thereby causing a significant energy loss.